1. Field of the Invention
This invention relates to an apparatus and method for use in drilling wells, and more particularly to an apparatus and method for controlling the operation of a motor used for operating a drill stem.
2. Background of the Related Art
Electric motors, especially direct current or dc motors, are commonly used to rotate a drill stem for drilling wells. The drill stem includes a drill pipe having a drill bit at one end. Silicon control rectifier (commonly known as "SCR" in the art) based control systems are typically used to supply regulated dc power to the motor. These systems have been used for many years in the industry and their operation is well known in the art. However, for the present purpose it is considered helpful to explain the prior art control systems by way of a functional block diagram, which is shown in FIG. 1.
Typical prior art control systems include an operator's console 30 and SCR drive and regulator 40. The SCR drive and regulator 40 supplies regulated power to the motor 10 via a conductor 54. The motor drives a gear box 14, which in turn rotates the drill pipe 18.
To start drilling, the operator sets the maximum permissible torque limit for the drill pipe by setting an upper current level that the SCR drive and regulator 40 may supply to the motor 10 by setting a torque control means 34 on the operator's console 30 and then increases the motor speed from its normal off-state speed, generally zero rpm, by adjusting a throttle or speed reference means 32 until a desired speed is set.
The SCR drive and regulator 40 receives the throttle and torque reference signals on lines 33 and 35 respectively and in response thereto supplies a constant or near constant voltage to the motor. The current supplied to the motor 10 (the motor input current) varies depending upon load placed on the motor, which is a function of the drill pipe torque. The drill pipe torque in turn depends upon borehole parameters, such as the borehole depth, borehole size, drill pipe weight, sustain formation borehole mud density, etc. As the borehole depth increases, additional pipe sections are connected to the drill pipe. The drill pipe always extends to the well depth, which continues to increase the drill pipe weight and thus the load on the motor. Individual drill pipe section are 20-50 feet long, thus the drill pipe has a pipe joint or a mechanical connection every few feet. These pipe joints from the weak points on the drill pipe.
The prior art systems operate acceptably as long as the motor is able to drive the drill stem within the parameters set by the operator. However, these prior art systems have severe drawbacks when the drill bit or drill pipe gets stuck or when the drill pipe torque increases beyond the drill pipe break point. When the drill pipe or drill bit gets stuck, it will continue to wind on itself about the point where it is stuck, causing the torque to continue to increase, thereby requiring the motor to demand increasing amounts of current so that it may maintain the preset speed.
The SCR drive and regulator 40 continues to increase the current to the motor until it reaches the current limit set by the operator. Thereafter, the voltage to the motor starts dropping as the motor speed drops until the drilling stops because the drill pipe is stuck.
In many cases, the operator becomes aware that the drill pipe is stuck only after the drill pipe has wound on itself or twisted to a substantial degree. Sometimes the operator finds out about this abnormal condition when the drilling shuts down. Under these abnormal conditions, operators frequently panic and either reduce the current limit too quickly or inadvertently reduce the throttle or motor speed reference before slowly lowering the upper current limit to zero. In either situation, the drill pipe will unwind too quickly, which may cause the pipe joints to become loose or even break. This will require the drill pipe to be pulled out from the borehole to repair the pipe joints or to fish-out the broken pipe portion left in the borehole, causing the drilling operation to stop for several hours, which can cost several thousand dollars per hour. More importantly, an operator error as noted above can cause serious injury to the personnel working on the drilling platform.
It is, therefore, highly desirable to have a system which will provide a warning soon after a drill pipe gets stuck during drilling or when the load requirements on the motor exceed a predetermined value and which system will controllably unwind the drill pipe to ensure that the drill pipe joints will not become loose or break and that the safety of the operators and equipment is not compromised.
The present invention provides a control system which addresses the above described problems with the prior art systems by providing an apparatus and method which detects these adverse conditions, provides audio and visual warnings to the operator, and controllably and safely unwinds the drill pipe.